A variety of mechanical devices are widely in use to mechanically couple two parts together or to bring one part into engagement with the other. These devices are used to transmit a given motion from one part to the other, to transform one motion to another, to hold the separate motions or to control a given motion. These devices quite often require that the state of engagement of parts is strictly regulated. For most devices, however, the regulation has hitherto had to be made only at the stage of assembling the parts. Once the parts are assembled to form a mechanical device, the re-adjustment of the state of engagement is generally not possible so that its utility is largely limited.